Superconducting analog-to-digital (ADC) converters are based on the ultra fast switching times of Josephson junction (JJ) comparators. This permits fast sampling rates of tens of gigahertz, leading to some of the fastest circuits in any electronic technology, for both Nyquist-rate ADCs and oversampling (sigma-delta and delta type) ADCs.
One well-known class of analog-to-digital converters is based on oversampling, in which a single-bit quantizer with feedback is used to generate a fast bit sequence that can accurately represent an analog input signal in the band of interest. Superconducting circuits based on Josephson junctions, in configurations known as rapid single-flux-quantum (RSFQ) logic, can switch on the picosecond timescale, leading to high sampling rates.
Sigma-delta and delta type ADCs have already been reported in superconducting electronics. In some of these ADCs an explicit feedback loop was avoided making use of a special feature of Josephson circuits, known as implicit feedback. However, implicit feedback hitherto was limited to only a single stage of oversampled ADCs.